1. Technical Field
The present invention relates to a liquid ejecting apparatus represented by a facsimile machine and a printer.
2. Related Art
Hereinafter, an ink jet printer as an example of a liquid ejecting apparatus is described. The ink jet printer has a supporting member (also referred to as platen) at a position opposed to an ink jet recording head. Further, the ink jet printer is configured such that a recording sheet is supported by the supporting member so as to define a distance between the ink jet recording head and the recording sheet.
In recent ink jet printers, size of ink droplets is progressively reduced in order to improve recording quality more preferably. For example, size of ink droplets is reduced to approximately several pl, for example. Therefore, mass of the ink droplets is significantly small and even when ink droplets are discharged from an ink jet recording head onto a recording sheet, some ink droplets do not land on the recording sheet and float as ink mists. This causes various types of problems. In addition, in so-called borderless recording, since ink droplets are also discharged onto regions beyond edges of a recording sheet, the above mist floating is caused more significantly. Note that in the borderless recording, recording is performed in a state where margins are not set on four sides of the recording sheet.
Then, as an existing technique, the following technique has been proposed as disclosed in JP-A-2007-118321 and JP-A-2007-118318. That is, potential differences are set among an ink jet recording head, a recording sheet and a supporting member so as to generate an electric field. Therefore, Coulomb's force is made to act on ink droplets so as to attract the ink droplets to a recording sheet.
Transportation of Recording Sheet
In recent ink jet printers, processing speed is made faster particularly for a business purpose and transportation speed of a sheet is largely increased in comparison with that in the past printers along with the increased processing speed. Further, in a so-called serial type ink jet printer, since head scanning (recording) is executed while a sheet is stopped, the sheet transportation speed is required to be further increased in order to prevent reduction in throughput. Note that in the serial type ink jet printer, recording is performed while an ink jet recording head moves in a direction perpendicular to a sheet transportation direction.
However, it has been found that the following problems arise in connection with such high-speed transportation of a recording sheet. Paper powder generated when a recording sheet is cut adheres to end portions (edges) of the recording sheet. In this case, if potentials among three components of the recording sheet, a supporting member (platen) and an ink jet recording head (hereinafter, integrally referred to as “recording portion constituent components”) are not controlled, the paper powder adhered to the recording sheet flies toward and adhere to the ink jet recording head by electric fields generated among the recording portion constituent components. In particular, when the recording sheet is transported at high speed, vibration or impact at the time of the sheet transportation is increased. Accordingly, flight of the paper powder is caused more significantly.
Further, friction charging or separation charging is significantly caused accompanied with friction between recording sheets accommodated in a sheet cassette, slide contact or contact between constituent components on a sheet transportation path (for example, an edge guide, a transportation roller, and the like) and the recording sheet. That is, the recording portion constituent components are significantly charged. As a result, electric fields formed among the recording portion constituent components are intensified, and charging of the paper powder itself is also intensified. This increases Coulomb's force acted on the paper powder so that the paper powder adheres to the ink jet recording head more significantly.
Further, even when the paper powder itself is not charged, if flown paper powder is placed in an electric field, bias of charges is caused in the paper powder due to dielectric polarization or electrostatic induction. Therefore, the paper powder is attracted to the side of the ink jet recording head.
FIG. 10 is a descriptive view for pointing out the problem. In FIG. 10, a reference numeral 160 indicates an ink jet recording head, a nozzle plate 160a, a supporting member (platen) 170, and a rib 170a formed on the supporting member 170 are illustrated. Further, a reference symbol P indicates a recording sheet, a symbol Pe indicates a sheet end portion, and a symbol d indicates paper powder. In addition, “+” and “−” each of which is surrounded by a circle indicate charging polarities.
The recording sheet P is electrically neutralized with a neutralization blush or the like. Therefore, the paper powder d adhered to the recording sheet P is not charged. However, when the nozzle plate 160a is positively charged and the supporting member 170 is negatively charged (as an example), charges are generated in the paper powder d as follows. That is, negative charges are generated in the paper powder d at the side of the nozzle plate and positive charges are generated in the paper powder d at the side of the supporting member due to dielectric polarization or electrostatic induction, as shown in an enlarged paper powder d in a circle. Note that the dielectric polarization is caused in a case where the paper powder d has a dielectric property and the electrostatic induction is caused in a case where the paper powder d has a conductive property. Therefore, the paper powder d is attracted to both the sides of the nozzle plate 160a and the supporting member 170.
If the paper powder adheres to the ink jet recording head, the paper powder directly closes a nozzle opening, or the paper powder moves to the nozzle opening when a nozzle surface is cleaned (wiped). This causes missing dots.
In addition to the above problem that the paper powder physically closes a nozzle opening, the following problem may be caused. That is, loading filler such as calcium carbonate forming the paper powder react with water content of ink so as to increase viscosity. Therefore, the loading materials inhibit vibration of meniscus of the nozzle opening and interfere with discharging of ink droplets in some case. Accordingly, it is extremely important to prevent the paper powder from adhering to an ink jet recording head in order to obtain appropriate recording quality in an ink jet printer.
In the above JP-A-2007-118321 and JP-A-2007-118318, the following technique has been proposed as described above. In the technique, potential differences are set among the ink jet recording head, the recording sheet, and the supporting member (recording portion constituent components) to generate electric fields. Then, Coulomb's force is made to act on ink droplets so as to attract the ink droplets to the recording sheet. Accordingly, it is considered that paper powder is attracted to the side of the recording sheet by controlling an electric field so as to prevent the paper powder from adhering to the ink jet recording head, if the paper powder is treated as the same triboelectric series as the ink droplets.
However, cellulose fibers and loading materials forming the paper powder are positively or negatively charged easily in terms of triboelectric series. Accordingly, if paper powder is tried to be prevented from flying toward the side of the ink jet recording head by forming an electric field in a specific direction among the recording portion constituent components, paper powder charged in an opposite polarity cannot be prevented from flying to the side of the ink jet recording head.
In JP-A-2003-165230, a recording apparatus having the following configuration is described. In the configuration, in order to prevent paper powder, dusts, or the like from adhering around a nozzle portion of an ink jet recording head as one of purposes, an air duct is provided around a nozzle plate and humidified air is ejected from the air duct at the time of recording and waiting for recording. However, the configuration causes increases in size of the apparatus and cost because the configuration is complex. Further, the configuration may result in an opposite effect of causing a risk that paper powder is made to adhere to the recording head due to airflow.
Further, in JP-A-2008-213255, a technique in which paper powder is collected by a paper powder collection member having a changing property is described. However, in the technique, paper powder cannot be necessarily collected effectively due to the above-described problem relating to the opposite polarity. Further, a problem relating to a method of processing (removing) paper powder deposited on the paper powder collection member arises in the technique. In particular, in a state where a large amount of the paper powder is deposited, there arises a risk that the paper powder scatters around even with little vibration or impact. This causes a problem on long-term maintenance of performance.